Sodium Pumps Mediate Activity-Dependent Changes in Mammalian Motor Networks

Abstract

Ubiquitously expressed sodium pumps are best known for maintaining the ionic gradients and resting membrane potential required for generating action potentials. However, activity- and state-dependent changes in pump activity can also influence neuronal firing and regulate rhythmic network output. Here we demonstrate that changes in sodium pump activity regulate locomotor networks in the spinal cord of neonatal mice. The sodium pump inhibitor, ouabain, increased the frequency and decreased the amplitude of drug-induced locomotor bursting, effects that were dependent on the presence of the neuromodulator dopamine. Conversely, activating the pump with the sodium ionophore monensin decreased burst frequency. When more “natural” locomotor output was evoked using dorsal-root stimulation, ouabain increased burst frequency and extended locomotor episode duration, whereas monensin slowed and shortened episodes. Decreasing the time between dorsal-root stimulation, and therefore interepisode interval, also shortened and slowed activity, suggesting that pump activity encodes information about past network output and contributes to feedforward control of subsequent locomotor bouts. Using whole-cell patch-clamp recordings from spinal motoneurons and interneurons, we describe a long-duration (∼60 s), activity-dependent, TTX- and ouabain-sensitive, hyperpolarization (∼5 mV), which is mediated by spike-dependent increases in pump activity. The duration of this dynamic pump potential is enhanced by dopamine. Our results therefore reveal sodium pumps as dynamic regulators of mammalian spinal motor networks that can also be affected by neuromodulatory systems. Given the involvement of sodium pumps in movement disorders, such as amyotrophic lateral sclerosis and rapid-onset dystonia parkinsonism, knowledge of their contribution to motor network regulation also has considerable clinical importance.SIGNIFICANCE STATEMENT The sodium pump is ubiquitously expressed and responsible for at least half of total brain energy consumption. The pumps maintain ionic gradients and the resting membrane potential of neurons, but increasing evidence suggests that activity- and state-dependent changes in pump activity also influence neuronal firing. Here we demonstrate that changes in sodium pump activity regulate locomotor output in the spinal cord of neonatal mice. We describe a sodium pump-mediated afterhyperpolarization in spinal neurons, mediated by spike-dependent increases in pump activity, which is affected by dopamine. Understanding how sodium pumps contribute to network regulation and are targeted by neuromodulators, including dopamine, has clinical relevance due to the role of the sodium pump in diseases, including amyotrophic lateral sclerosis, parkinsonism, epilepsy, and hemiplegic migraine.

Sodium pump regulation of locomotor control circuits

“…Expression of ␣3 is also found in interneurons, and in our experiments, we specifically focused on ␣3 expression in one interneuron type, the cholinergic pitx2 cells (Zagoraiou et al 2009). We found ␣3 expression in around half of this population, which broadly matches the number of pitx2 neurons found to display the usAHP (Picton et al 2017). This is also similar to findings of previous studies in rats, which documented an activity-dependent, pump-mediated hyperpolarization in around half of cultured spinal interneurons (Darbon et al 2002(Darbon et al , 2003.…”

“…A similar heterogenous usAHP distribution is present in the neonatal mouse CPG ( Fig. 6; Picton et al 2017). For MNs, a very similar proportion to that in the tadpole (~40%) display the usAHP.…”

“…This proportion is similar to that in tadpole interneurons when cINs, aINs, and dINs are pooled. Although the identity of all the specific interneuron classes displaying a usAHP in neonatal mice is not yet known, one type of modulatory neurons, the cholinergic pitx2 class, was found to display a usAHP in around 60% of the population (Picton et al 2017).…”

“…This suggests a highly conserved, pump-mediated dynamic regulation of motor circuit function. In spinal motor circuits, the duration of a bout of locomotion is influenced by previous network activity if two bouts occur within about a minute of each other; a form of short-term motor memory (Picton et al 2017;Zhang and Sillar 2012;Zhang et al 2015). This motor memory relies on the presence of a pump-mediated ultraslow afterhyperpolarization (usAHP) of up to 10 mV in spinal neurons, which lasts for the same duration of approximately a minute.…”

Capsaicin-Induced Impairment of Functional Network Dynamics in Mouse Hippocampus via a TrpV1 Receptor-Independent Pathway: Putative Involvement of Na+/K+-ATPase

“…Instead, our data show that Cp-induced increase in the first-spike latency could be reversed by ouabain, demonstrating the involvement of the Na + /K + -ATPase in the Cpinduced reduction of AP firing rate and the subsequent reduction in gamma oscillations. Supporting our conclusion, Na + / K + -ATPase has been reported to regulate excitability in several neuron types, as well as rhythmic network activity in the CNS, working as a sensor of AP firing activity in neurons in which Na + /K + -ATPase function increases after prolonged or intense AP firing due to intracellular Na + accumulation [33,34,[51][52][53][54]. The reduction of excitability by activitydependent recruitment of the Na + /K + -ATPase has been studied in hippocampus by Gulledge and coworkers (2013) [34].…”